1. to become hard, rigid, or less flexible
2. to make something hard, rigid, or less flexible
3. to become tense or resistant in attitude or behavior

Examples

• The cold weather stiffens the clay, making it harder to work with.
• She stiffens her resolve before entering the difficult meeting.
• The fabric stiffens when you add starch to it.
• His back stiffens after sitting at the desk all day.
• The mixture stiffens as it cools down.
• He stiffens whenever someone mentions his past mistakes.
• The wind stiffens the flag against the pole.